1. Field of the Invention
The present invention relates to a gallium arsenide phosphide-, mixed crystal-epitaxial wafer which is appropriate for the production of an array of light-emitting diodes having improved uniformity in the brightness thereof.
2. Description of the Related Arts
In a printer system in which electrostatic latent images are produced by means of optically image-forming characters or figures on a photosensitive layer consisting of electrostatic photosensitive material, a toner is applied on the photosensitive layer, and the electrostatic latent images are then copied on the paper or the like. This kind of system features low noise and a high speed and is used as a printer for a computer or word processor. Hereinafter, this printer system will be referred to as an electrostatic printer.
Conventionally, a gas laser, such as helium-neon laser, or a semiconductor laser, is used as the light source of an electrostatic printer. The former light source is disadvantage by its large size, and although the latter light source is small in size it nevertheless has a drawback. Namely a semiconductor laser of continuous oscillation type practically used at present exhibits an oscillation wavelength of 780 nm or longer. On the other hand, the photosensitive materials, such as cadmium sulfide (CdS), and selenium (Se), exhibit a highest sensitivity in a short wavelength region of 700 nm or shorter. The oscillation wavelength of semiconductor laser do not, therefore, match the wavelength at which the highest sensitivity of the photosensitive materials is exhibited. Consequently a high efficiency is not obtained.
In addition to the above drawback, the semiconductor laser is not easily produced because of a complicated structure, exhibits a short life, and is expensive (Bulletin for Electron Photograph Society Vol. 23, No. 2, pp 47-55 (1984)).
Light emitting diodes have recently attracted attention, since, the emission wavelength thereof can be freely selected from the infrared region to a visible region, and thus the matching between the emission wavelength and the wavelength at which the photosensitive materials exhibit the highest sensitivity can be easily attained. In addition, the life of light emitting diodes is long, and their structure is simple and compact.
When the laser is used as a light source, since it has a satisfactory brightness, one laser or several lasers are scanned over the electrostatic photosensitive layer to generate latent images. The brightness of a light emitting diode is generally lower than that of a semiconductor lazer and, therefore, an array of light emitting diodes is constituted for providing the light source of an electrostatic printer. In this array, for example, from sixteen to thirty two light emitting diodes per mm are aligned in one row having a length of several millimeters, and a number of such rows are arranged along the photosensitive layer, so as to form arrays of light emitting diodes. The discrete light emitting diodes which constitutes the arrays are controlled independently from one another, so as to form latent images.
For producing the arrays of light emitting diodes (hereinafter referred to as "LED arrays"), not the so called film hybrid structure, in which requisite numbers of discrete light emitting diodes are mounted on a ceramic substrate and are electrically connected with each other, is employed, but the monolithic structure, in which the requisite number of minute light emitting diodes are formed on one substrate of single crystalline gallium arsenide, gallium phosphide, or the like, is generally employed. This is because, the light emitting diodes can be highly integrated in the monolithic structure, and this facilitates the size reduction of a light source and provides an excellent mass productivity.
Gallium arsenide phosphide (GaAs.sub.1-x P.sub.x, 0&lt;x&lt;1), mixed crystal-epitaxial wafer, which is appropriate for the emission of light having a wavelength of from 700 to 600 nm, at which the electrostatic photosensitive material exhibits the highest sensitivity, is used as the semiconductor material for producing the LED arrays used for an electrostatic printer. The gallium arsenide phosphide (GaAs.sub.1-x P.sub.x, 0&lt;x&lt;1) is hereinafter referred to as the GaAsP, and "x" is hereinafter referred to as the mixed crystal ratio.
The conventional GaAsP epitaxial wafer has the following structure. As the single crystalline substrate, single crystalline gallium arsenide (GaAs) or gallium phosphide (GaP) is generally used. A graded layer is formed on the single crystalline substrate. In this layer, the mixed crystal ratio continuously changes from x=0 (i.e., GaAs) or x=1 (i.e., GaP) to a predetermined value (x.sub.1) of a constant layer which is formed on the graded layer. The continuous change of the mixed crystal ratio from x=0 or x=1 to X.sub.1 is intended to prevent the generation of crystal defects, such as dislocation and the like, due to differences in the lattice constant between the substrate and the constant layer. On the graded layer, the constant layer is formed.
The mixed crystal ratio of this layer is determined, when an LED is produced using the GaAsP epitaxial layer, so as to obtain the light emission of a desired wavelength. A pn junction is formed in the constant layer when producing an LED, and the obtained LED emits the light having a wavelength corresponding to the band gap of this layer.
When the LED arrays are used as the light source of an electrostatic printer, the brightness of each LED included in the LED arrays is required to be within .+-.20% of the average brightness value of the total number of LEDs contained in the LED arrays, since otherwise the brightness distribution results in a distribution of the printed image density. When the LED arrays are produced by using conventional GaAsP epitaxial wafers, a brightness distribution within .+-.20% is obtained at a very low yield of the LED arrays.